This document provides an introduction to medical instrumentation and bioamplifiers. It discusses how medical instrumentation measures and monitors physiological signals in the body using sensors. The key components of a biomedical instrumentation system are described including the measurand, sensor/transducer, signal conditioner, display, and data storage. It then focuses on bioamplifiers, explaining the types (differential, operational, instrumentation, isolation), their characteristics, and how they are used to amplify weak biopotential signals from the body while maintaining signal integrity.

1. UEEG002
Medical Instrumentation –
Introduction,Bioamplifiers
D.Poornima,AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore.

2. Introduction
What is Medical Instrumentation??
• Application of biomedical engineering
• Focuses on the devices and mechanics used to measure, evaluate,
and treat biological systems.
• Uses multiple sensors to monitor physiological characteristics of a
human or animal.
• Helps physicians to diagnose the problem and provide treatment,
to measure biological signals and to design a medical instrument.
• Originated as a necessity to constantly monitor vital signs of
Astronauts during NASA's Mercury, Gemini, and Apollo missions

3. Components of Biomedical Instrumentation
System
• Any medical instrument consists of the following functional
basic parts
– Measurand
– Sensor / Transducer
– Signal Conditioner
– Display
– Data Storage and
Data Transmission

4. • Measurand
– The measurand is the physical quantity,property or condition that instrumentation
system measures. Human body acts as the source for measurand, and it generates
bio-signals.
Example: Blood pressure (Internal)
Electrocardiograms (on the body surface)
Blood or Biopsy (Emanate from the body)
• Sensor / Transducer
– Converts one form of energy to another form usually electrical energy.
– The transducer produces a usable output depending on the measurand.
– The sensor is used to sense the signal from the source. It is used to interface
the signal with the human
Example: Piezoelectric signal which converts mechanical vibrations into the
electrical signal

5. • Signal Conditioner
– Used to convert the output from the transducer into an electrical
value.
– The instrument system sends this quantity to the display or recording
system.
– signal conditioning process includes amplification, filtering, analogue to
digital and Digital to analogue conversions.
– Signal conditioning improves the sensitivity of instruments.
• Display
– Provides a visual representation of the measured parameter or
quantity.
Example: Chart recorder, Cathode Ray oscilloscope (CRO).
– Sometimes alarms are used to hear the audio signals.
Example: Signals generated in Doppler Ultrasound Scanner used for
Fetal Monitoring.

6. • Data Storage and Data Transmission
– Data storage is used to store the data and can be used for future reference.
– Recent days Electronic Health records are utilized in hospitals.
– Data transmission is used in Telemetric systems, where data can be transmitted from
one location to another remotely.

7. Bioelectric Potential
• Human body has many subsystems inside it like
– Cardiovascular system
– Respiratory system
– Nervous system
– Digestive system
• The systems in the human body generate their own monitoring signals when
they carry out their functions.
• These signals provide useful information about their function.
• These signals are bioelectric potentials associated with nerve conduction,
brain activity, heartbeat, muscle activity and so on.
• Bioelectric potentials are actually ionic voltages produced as a result of
electro chemical activity of certain cell.
• Transducers are used to convert these ionic potentials in to electrical signals

8. Cell Structure

9. • Certain types of cells within the body, such as nerve and muscle cells are
encased in a semi permeable membrane.
• This membrane permits some substances to pass through while others
are kept out.
• Surrounding the cells of the body are the body fluids.
• These fluids are conductive solutions containing charged atoms know as
ions.
• The principle ions are sodium (Na+) Potassium (K+) and chloride (Cl-).
• The membrane of excitable cells permits entry of
Potassium (K+) and chloride(C-) ions but blocks
the entry of sodium (Na+) ions.

10. Resting Potential
• So potential inside the cell is more negative than outside cell.
• This membrane potential is called Resting potentials.
• This potential is measured from inside the cell with respect to body fluids.
• So resting potential of a cell is negative.
• This resting potential ranging from -60mv to -100 mv.
• Cell in the resting state is called polarized cell.

11. Action Potential [Depolarization]
• When a section of a cell membrane is excited by the flow of
ionic current or by some form of externally applied energy,
the membrane allows some Na+ and try to reach some
balance of potential inside and outside.
• Same time some K+ goes outside but not rapidly like
sodium.
• As a result, the cell has slightly Positive potential on the
inside Due to the imbalance of the Potassium ions.
• This potential is known as “action potential” and is
approximately +20 mV

12. • A cell that has been excited and that displays an action potential is said to
be depolarized
• The process from resting to action potential is called depolarization.
• Action potential is always positive inside.

13. Reploarization
• The process of the cell coming to
depolarised state from polarised state

14. Characteristics of Resting and Action Potential

15. Bio potential Amplifiers
– Very important part of modern medical instrumentation
– To amplify biopotentials which are generated in the body at
low levels with high source impedance
– are required to increase signal strength while maintaining
fidelity
• Functions of Biopotential Amplifiers
– To take a weak bio-potential and increase its amplitude so
that it can be processed, recorded or displayed
– To amplify voltage, power and current.
– Sometimes used to isolate the load from the source current
gain only

16. • Characteristics of Biopotential Amplifiers
– High input impedance
– Low output impedance
– The biopotential amplifier must be sensitive to important
frequency components of the biosignal
– Biopotential amplifiers have a gain of 1000 or greater
– Most biopotential amplifiers are differential
– High common mode rejection ratio.

17. Types of Bio Amplifiers
1. Differential Amplifier
2. Operational Amplifier
3. Instrumentation Amplifier
4. Isolation Amplifier

18. Differential Amplifier
• Type of electronic amplifier that amplifies the difference between two
input voltages but suppresses any voltage common to the two inputs.
• A differential amplifier is an analog circuit with two inputs (V1 and V2)
and one output (V0) in which the output is ideally proportional to the
difference between the two voltages.
• The formula for a simple differential amplifier can be expressed:
• V0 is the output voltage
• V1 and V2 are the input voltages
• Ad is the gain of the amplifier (i.e. the differential amplifier gain)

19. • When V1 = V2, V0 is equal to zero, and hence the output voltage is
suppressed.
• But any difference between inputs V1 and V2 is multiplied
by the differential amplifier gain Ad.
• It is also known as a difference amplifier
– the difference between the input
voltages is amplified.

20. Operational Amplifier
• Op amp is basically a multistage amplifier in which several amplifier stages are
interconnected to each other in a very complicated manner.
• Its internal circuit consists of many transistors, FETs and resistors. All this occupies
a very little space.
• It is packed in a small package and is available in the Integrated Circuit (IC)form.
• The term Op Amp is used to denote
an amplifier which can be configured
to perform various operations like
amplification, subtraction,
differentiation, addition, integration etc

21. • Applying two signals, one at the inverting and another at the non-
inverting terminal, op-amp will amplify the difference between
them
• The difference between two input signals is called as the
differential input voltage.
• The equation below gives the output of an operational amplifier.
• VOUT is the voltage at the output terminal of the op-amp.
• AOL is the closed-loop gain for the given op-amp and is constant (ideally)
• The feedback is negative if the feedback path feeds the part of
the signal from the output terminal back to the inverting (-)
terminal.
• We use negative feedback to the op-amps used as amplifiers

22. Chopper Amplifier
• This circuit uses a chopping device which converts slowly varying dc
to an alternating form with amplitude proportional to the input
direct current and phase depends on the polarity of original signal.
• The ac voltage is then amplified by an ac amplifier whose output is
then rectified back to get an amplified direct current.
• Figure shows single ended chopper stabilizer amplifier.
• Aim of this circuit is to avoid the dc offset voltage present in the
output signal.
• For that purpose first convert the dc signal into ac using a chopper

24. Components
• Low Pass Filter
– The low frequency components derived from the input signal by passing it through low
pass filter R2C2 and R2
• Chopper
– The output of LPF is then chopped using a transistor switch w.r.t a carrier signal from
the oscillator.
• Demodulator
The original signal recovered in demodulator which is again applied to second stage of
amplification.
• Low Pass Filter
– Before given to the amplifier low frequency components again derived using LPF
• Second Stage of Amplification (A2)
– At the input of A2 an HPF C1R1 is used to derive the high frequency signals.
– This is to reduce the dc offset and drift of second amplifier A2.

25. Instrumentation Amplifier
• Commonly used Bio-potential Amplifier is Differential amplifier. But it has
some limitations.
• These limitations overcome with the availability of improved version of
differential amplifier called Instrumentation Amplifier
• High gain and the high input impedance are attained with an instrumentation
amplifier.

26. • Usually, a 3-amplifier and 7 resistor setup forms the
instrumentation amplifier circuit.
• The output from the transducer is given as input to the
instrumentation amplifier.
• Before the signal goes to the next stage, a special amplifier is
required with high CMRR, high input impedance and to avoid
loading effects.
• Two buffer amplifiers A1 and A2 connected to a differential
• amplifier A3.
• Op-amp A3 with four equal resistors R form a differential
• amplifier with gain 1.
• Rg = used to set the gain using the formula

28. • Advantage of Chopper Amplifier
 Insensitivity to component changes due to ageing, temperature change, power
supply variation, or other environmental factors.
 Small offset voltage
 Used to amplify small dc signals of few microvolts.

29. Isolation Amplifier
• Used for protection against leakage currents
• Isolation between input and output
• Isolation between different supply voltages
and grounds
• Three methods used in the design
– Transformer Isolation
– Optical Isolation
– Capacitive Isolation

30. • Transformer Isolation
– Uses Frequency modulated or a pulse width modulated carrier signal
– Uses internal DC-DC converter
comprising of a 20kHz oscillator,
transformer, rectifier, and filter
to supply isolated

31. • Optical Isolation
– Can also be achieved by optical means
– Patient is not connected electrically to hospital line or ground
– Separate battery supplies power to the patient circuit
– Patient bio signal is converted into a light by a light source
– On output side, photo transistor
converts light signal again to
electrical signal

32. • Capacitive Isolation
– Uses digital encoding of the input voltage and frequency modulation to
send the signal across a differential capacitive barrier
– Separate power supply needed
on both sides of the barrier